fix: cloning large repo's failed with fatal: early EOF

[alecthomas]

This only manifested with --bare --mirror because the clones are so large, due to including all refs.

Also switched to using httputil.ReverseProxy, which is more robust.

~/dev/cachew $ rm -rf git-source ; time git clone --bare --mirror http://127.0.0.1:8080/git/github.com/git/git.git git-source
Cloning into bare repository 'git-source'...
remote: Enumerating objects: 807474, done.
remote: Counting objects: 100% (8533/8533), done.
remote: Compressing objects: 100% (7947/7947), done.
error: RPC failed; curl 18 transfer closed with outstanding read data remaining
error: 297 bytes of body are still expected
fetch-pack: unexpected disconnect while reading sideband packet
fatal: early EOF
fatal: fetch-pack: invalid index-pack output
git clone --bare --mirror http://127.0.0.1:8080/git/github.com/git/git.git   5.26s user 1.17s system 21% cpu 30.509 total

Once the cache is populated, cloning git takes half the time:

~/dev/cachew $ rm -rf git-source ; time git clone http://127.0.0.1:8080/git/github.com/git/git.git git-source
Cloning into 'git-source'...
remote: Enumerating objects: 403536, done.
remote: Counting objects: 100% (756/756), done.
remote: Compressing objects: 100% (363/363), done.
remote: Total 403536 (delta 532), reused 498 (delta 393), pack-reused 402780 (from 4)
Receiving objects: 100% (403536/403536), 282.29 MiB | 19.06 MiB/s, done.
Resolving deltas: 100% (305003/305003), done.
git clone http://127.0.0.1:8080/git/github.com/git/git.git git-source  18.61s user 2.65s system 96% cpu 21.940 total
~/dev/cachew $ rm -rf git-source ; time git clone http://127.0.0.1:8080/git/github.com/git/git.git git-source
Cloning into 'git-source'...
remote: Enumerating objects: 403536, done.
remote: Counting objects: 100% (403536/403536), done.
remote: Compressing objects: 100% (95986/95986), done.
remote: Total 403536 (delta 305001), reused 403485 (delta 304954), pack-reused 0 (from 0)
Receiving objects: 100% (403536/403536), 282.15 MiB | 117.21 MiB/s, done.
Resolving deltas: 100% (305001/305001), done.
git clone http://127.0.0.1:8080/git/github.com/git/git.git git-source  18.11s user 2.34s system 206% cpu 9.916 total